In recent years, a portable electronic device such as a smart phone or a tablet has a display device with a touch sensor. The touch sensor detects contact or close proximity of a conductive material (hereafter “contact” and “close proximity” is generically called “touch”). The conductive material includes a finger, a stylus, and so forth. Such an electronic device displays on a screen a menu image including many icons, for example, and determines which icon is touched in order to recognize which action the electronic device should take. Consequently, a user can operate an electronic device, without using such an input device as a keyboard, a mouse, or a keypad.
A touch sensing system includes an optical type system, a resistance type system, a capacitance type system, and others. Among them, the capacitance type system is widely used since it is comparatively simple in structure and little in energy consumption. A touch sensor employing a capacitance type system detects a touch using a fact that an electrode will change in its capacitance when an electric conductor approaches the electrode (i.e., an electrode will increase in capacitance when an electric conductor approaches the electrode).
It is possible to attach a touch panel with such a touch sensor on a screen of a display device. In recent years, however, a touch panel is built into a display device in many cases. With the progress of making thinner a display device with a touch panel, electrodes of a touch sensor is closer to various wirings for driving the display device to display an image or the wirings is much closer to one another. As a result, parasitic capacitance may be connected among them. If parasitic capacitance is connected between a driving electrode and a wiring in a capacitance type touch sensor, a drive signal will deteriorate in waveform and is delayed in transmission, whereby touch detection deteriorates in accuracy or detection time is long. In this way, a capacitance type touch sensor is easily affected by parasitic capacitance.